Inhibitors of hydroxyphenylpyruvate dioxygenase and an assay for identification of inhibitors

ABSTRACT

There are described a method for enriching the anabolic p-hydroxyphenylpyruvate dioxygenases from plant tissues and a method by means of which the enzymatic activity of the enzyme can be measured in a simple manner without complete purification of the enzyme being necessary. 
     The invention furthermore describes an assay system for identifying inhibitors of p-hydroxyphenylpyruvate dioxygenase from plants, in which an enriched HPPD from plants is incubated with a test substrate to be examined and the enzymatic activity of the enzyme is determined in comparison with the activity of the uninhibited enzyme. 
     The invention furthermore describes the novel herbicidal active substances of the formula (I) ##STR1## which were found using the assay method according to the invention.

DESCRIPTION

This application is a division of application Ser. No. 08/369,875, filed on Jan. 6, 1995, which in turn is a continuation of application Ser. No. 08/200,741 filed on Feb. 23, 1994, now abandoned.

p-Hydroxyphenylpyruvate dioxygenase (HPPD; p-hydroxyphenylpyruvate: oxygen oxidoreductase, EC 1.13.11.27) in plant tissues is an enzyme central to the biosynthesis of the quinoid- compounds derived from the amino acid tyrosine, such as plastoguinones or tocopherols. During the biosynthesis of quinone, tyrosine is first transaminated by a transaminase to give p-hydroxyphenylpyruvate, which is then decarboxylated and hydroxylated by the enzyme HPPD in a complex sequence. The product of this reaction is homogentisic acid. These first two steps of quinone biosynthesis can analogously also be detected in animal tissues and microorganisms. While, in plants, the homogentisic acid formed can be reacted to give plastoquinones and tocopherols, however, it is an intermediate in the catabolism of the amino acid tyrosine in animals and microorganisms. Plastoquinones and tocopherols are essential structures for plants. Inhibitors of the biosynthesis of plastoquinones and tocopherols should therefore be potential herbicides.

Catabolic p-hydroxyphenylpyruvate dioxygenases have previously been purified and characterized from animal tissues (Lindstedt, S. and Odelhog, B. (1987) Meth. Enzymol. 142, 139-142) and microorganisms (Lindstedt, S. and Odelhog, B. (1987) Meth. Enzymol. 142, 143-148). (Anabolic) p-hydroxyphenylpyruvate dioxygenases from plants, in contrast, have been described in the literature (Fiedler, E., Soll, J. and Schultz, G. (1982) Planta 155, 511-515), but no method for enriching the enzyme has been described to date.

Specifically, there has been no information to date in the literature on simple methods of measuring the plant enzyme which, of course, are indispensible for purification of the enzyme and detecting inhibitors of the enzyme.

The method described by Fiedler et al. is relatively complicated and does not allow the quantitative assays to be carried out which are required for identifying potential herbicides.

What follows describes a method for enriching the anabolic p-hydroxyphenylpyruvate dioxygenases from plant tissues and a method by means of which the enzymatic activity of the enzyme can be measured in a simple manner without complete purification of the enzyme being necessary.

The invention therefore relates to:

A method for enriching an anabolic p-hydroxyphenylpyruvate dioxygenase from plant cells, which comprises isolating the enzyme directly from a buffer in which the cells were homogenized.

The invention furthermore relates to an assay system for identifying inhibitors of p-hydroxyphenylpyruvate dioxygenase from plants, which comprises incubating an enriched HPPD from plants with a test substrate to be examined and determining the enzymatic activity of the enzyme in comparison with the activity of the uninhibited enzyme.

In particular, the invention relates to a method in which the plant tissue or cells are homogenized in extraction buffer, the extract is filtered, the supernatant is subjected to fractional ammonium sulfate precipitation, and the precipitate formed is redissolved.

Suitable extraction buffers are the extraction buffers conventionally used for plant cells, in particular the extraction buffer which is composed as follows:

20 mM phosphate buffer pH 7.0;

0.14M KCl;

0.1 mg/ml glutathione and

1% of insoluble polyvinylpyrrolidone.

Suitable test substrates are all compounds which are potential HPPD inhibitors. These potential inhibitors are preferably stucturally similar to the natural substrate of HPPD. However, other substances which do not bind in the active center of the enzyme, but which inhibit the enzyme in a different manner, can also be used.

The enzymatic activity can be determined by means of the methods known from the literature (see Bergmeyer, H. U., Methoden der enzymatischen Analyse Methods in enzymatic analysis!, Volumes 1 and 2, Verlag Chemie, Weinheim, (1974) and Suelter, C. H., Experimentelle Enzymologie: Grundlagen fur die Laborpraxis Experimental Enzymology: Fundamentals of Laboratory Practice!, Fischer Stuttgart (1990)).

A modified form of the method for measuring catabolic HPPDs from human liver has been described by Lindblad (Lindblad, B. (1971) Clin. Chim. Acta 34, 113-121). End-point measurements are used in this case for detecting ¹⁴ C--CO₂, which is liberated from ¹⁴ C-p-hydroxyphenylpyruvate by the enzymatic activity of HPPD. If HPPD inhibitors are present in the reaction batch, the enzyme reaction and hence the liberation of CO₂ are suppressed. This assay method allows inhibitors of the enzyme to be found.

It is preferred to carry out the assay in such a way that the enzymatic activity of the HPPD is started up after preincubation of the enriched HPPD with the potential inhibitor by adding the radiolabeled ¹⁴ C-p-hydroxyphenylpyruvate, stopping the reaction after a suitable incubation time, and measuring the enzymatic activity indirectly via the radioactivity which has been liberated.

It is self-evident that the assay according to the invention can also be carried out with the purified enzyme. The enzyme can be further enriched in a manner known per se by means of chromatography on anion exchangers, such as, for example, Q-Sepharose, by Pharmacia, followed by gel permeation chromatography, such as, for example, Superdex 200, by Pharmacia, but this is not necessary for carrying out the assay.

Surprisingly, a new class of HPPD inhibitors has now also been identified using this assay system. They are herbicides from the group of the 2-benzoylcyclohexane-1,3-diones. This is surprising since an entirely different mechanism of action is suggested in the literature for this compounds class on the basis of its herbicidal symptoms (they lead to bleaching of the plants). It is assumed that, analogously to other bleaching herbicides, they inhibit phytoene desaturase (Soeda, T. and Uchida, T. (1987) Pestic. Biochem. Physiol. 29, 35-42; Mayonada, D. J., Hatzios, K. K., Orcutt, D. M. and Wilson, H. P. (1989) Pestic. Biochem. Physiol. 35, 138-145). It was possible to demonstrate that this compounds class, while not inhibiting phytoene desaturase, does inhibit HPPD; under the assay conditions, 4.5×10⁻⁸ M 2-(2-chloro-4-methanesulfonylbenzoyl)-1,3-cyclohexanedione (SC-0051, a typical representative of this class of herbicides) showed a 50% inhibition on the anabolic plant HPPD from maize (which corresponds to the IC50 value). Bleaching herbicides which have a different structure do not, in contrast, inhibit HPPD, but inhibit the enzyme phytoene desaturase.

However, the method according to the invention allows not only the activity of known herbicides to be demonstrated at the molecular level, but also novel, previously unknown herbicidal active substances which inhibit HPPD to be identified.

The invention therefore also relates to the novel herbicidal active substances found by the assay method according to the invention.

The invention therefore also relates to compounds of the formula (I) ##STR2## in which R¹ is H, halogen, OH, alkoxy, CN, NO₂ or haloalkyl,

R² is H, halogen, OH, alkyl, alkoxy, haloalkyl, haloalkoxy or (alkoxy)-carbonyl,

R³ is H, halogen, OH, CN, NO₂, haloalkyl, haloalkoxy or R⁴ S(O)_(m) --, where R⁴ is alkyl and m is zero, one or two,

Q is a radical selected from the group of the formula ##STR3## where R⁵, R⁶, R⁷, R⁸ and R⁹ independently of one another are H, halogen, OH, CN, NO₂, SO₃ H, SO₃ R⁴, SO₂ NR¹⁴ R¹⁵, COOH, COOR⁴, CONR¹⁴ R¹⁵, O--COOR⁴, O--COR⁴, -alkyl, alkoxy, haloalkyl or haloalkoxy,

R¹⁰, R¹¹ and R¹² independently of one another are H, halogen, alkyl, alkoxy, haloalkyl or haloalkoxy,

R¹³ is H, phenylsulfonyl, alkylsulfonyl or alkylcarbonyl, and

R¹⁴ and R¹⁵ independently of one another are H, alkyl, aryl or benzyl,

with the following provisos:

(i) R¹ to R³ or R⁵ to R⁹ are not simultaneously hydrogen;

(ii) R⁵, R⁷ and R⁹ are not simultaneously hydroxy;

(iii) when R³ is alkoxy then R⁵ or R⁹ is halogen and when R⁷ is alkoxy then R⁶ or R⁸ is not hydrogen; and

(iv) when R¹ is hydroxy then R⁵ and R⁹ are not hydroxy.

In the formula (I), the radicals alkyl, alkoxy, haloalkyl and haloalkoxy can in each case be straight-chain or branched. Alkyl radicals are, for example, methyl, ethyl, n- or i-propyl or n-, i-, t- or 2-butyl. Aryl embraces aromatic and heteroaromatic radicals, such as, for example, phenyl, 2- or 3-naphthyl or 2-, 3- or 4-pyridyl. Halogen is fluorine, chlorine, bromine or iodine.

The use according to the invention of compounds of the formula (I) in which

R¹ is H, halogen, OH, C₁ -C₃ -alkoxy, CN, NO₂ or C₁ -C₃ -haloalkyl,

R² is H, halogen, OH, C₁ -C₃ -alkyl, C₁ -C₃ -alkoxy, C₁ -C₂ -haloalkyl, C₁ -C₂ -haloalkoxy or (C₁ -C₂ -alkoxy)carbonyl,

R³ is H, halogen, OH, CN, NO₂, C₁ -C₃ -haloalkyl, C₁ -C₂ -haloalkoxy or R⁴ S(O)_(m) --, where R⁴ is C₁ -C₂ -alkyl and m is zero, one or two,

Q is a radical selected from the group of the formula ##STR4## where R⁵, R⁶, R⁷, R⁸ and R⁹ independently of one another are H, halogen, OH, CN, NO₂, SO₃ H, SO₃ R⁴, SO₂ NR¹⁴ R¹⁵, COOH, COOR⁴, CONR¹⁴ R¹⁵, O--COOR⁴, O--COR⁴, C₁ -C₄ -alkyl, C₁ -C₂ -alkoxy, C₁ -C₃ -haloalkyl or C₁ -C₂ -haloalkoxy,

R¹⁰, R¹¹ and R¹² independently of one another are H, halogen, C₁ -C₃ -alkyl, C₁ -C₂ -alkoxy, C₁ -C₃ -haloalkyl or C₁ -C₂ -haloalkoxy,

R¹³ is H, phenylsulfonyl, C₁ -C₂ -alkylsulfonyl or C₁ -C₂ -alkylcarbonyl, and

R¹⁴ and R¹⁵ independently of one another are H, C₁ -C₄ -alkyl, phenyl or benzyl,

is of particular interest.

Preferred is the use according to the invention of compounds of the formula (I) with at least one of the following characteristics:

R¹ is preferably hydrogen, hydroxyl, fluorine, chlorine, bromine, cyano, nitro, methoxy, ethoxy, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, 2,2,2-trifluoroethyl or pentafluoroethyl,

R² is preferably hydrogen, fluorine, chlorine, hydroxyl, propyl, ethyl, methyl, methoxy, ethoxy, difluoromethyl, trifluoromethyl, difluoromethoxy, trifluoromethoxy, methoxycarbonyl or ethoxycarbonyl,

R³ is preferably hydrogen, hydroxyl, fluorine, chlorine, bromine, cyano, nitro, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, 2,2,2-trifluoroethyl, pentafluoroethyl, difluoromethoxy, trifluoromethoxy, methylsulfonyl or ethylsulfonyl;

Q is preferably the radical ##STR5## where R⁵, R⁶, R⁷, R⁸ and R⁹ preferably and independently of one another are hydrogen, fluorine, chlorine, bromine, cyano, nitro, hydroxyl, hydroxysulfonyl, ethoxysulfonyl, ethylaminosulfonyl, methylaminosulfonyl, carboxyl, ethoxycarbonyl, methoxycarbonyl, ethylaminocarbonyl, methylaminocarbonyl, ethoxycarbonyloxy, methoxycarbonyloxy, methylcarbonyloxy, t-butyl, i-propyl, propyl, ethyl, methyl, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, methoxy, ethoxy, difluoromethoxy or trifluoromethoxy,

R¹⁰, R¹¹ and R¹² preferably and independently of one another are hydrogen, fluorine, chlorine, bromine, ethyl, methyl, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, methoxy, ethoxy, difluoromethoxy or trifluoromethoxy,

R¹³ is preferably hydrogen, phenylsulfonyl, methylsulfonyl, ethylsulfonyl, methylcarbonyl or ethylcarbonyl, and

R¹⁴ and R¹⁵ preferably and independently of one another are hydrogen, butyl, propyl, ethyl, methyl or benzyl.

The uses according to the invention of compounds of the formula (I) with a combination of the abovementioned preferred characteristics are also preferred.

The compounds of the abovementioned formula (I) can be prepared, for example, by reacting aromatic carboxylic acid chlorides of the formula (II) ##STR6## in which the substituents R¹, R² and R³ are as defined under formula (I), with aromatic or heteroaromatic compounds of the type H-Q, in which Q is as defined under formula (I), in an inert solvent in the presence of a Lewis acid.

The compounds (I) are preferably prepared in aprotic solvents, in particular in halogenated hydrocarbons, such as, for example, dichloromethane, trichloromethane, 1,2-dichloromethane or 1,1,2,2-tetrachloroethane, at from -20° C. up to the boiling point of the reaction mixture, in particular at from room temperature to 50° C., the reaction being carried out in the presence of a catalytically to stoichiometrically employed amount of Lewis acid, such as, for example, aluminum chloride, titanium tetrachloride or boron trifluoride.

A. PREPARATION EXAMPLES

a) 3-(2,4-Dichlorobenzoyl)-1-phenylsulfonylpyrrole (Table 1, Example No. 150)

4.0 ml of 2,4-dichlorobenzoyl chloride and 4.00 g of aluminum chloride are stirred for 10 minutes at room temperature in 50 ml of 1,2-dichloroethane. A solution of 5.10 g of phenylsulfonylpyrrole in 20 ml of 1,2-dichloroethane is added dropwise to the mixture, and this is stirred for 2 hours at room temperature. The mixture is poured into ice-water and extracted using dichloromethane. The organic phase is dried and evaporated. Recrystallization of the residue from a mixture of ethyl acetate and petroleum ether gives 8.3 g (89% of theory) of 3-(2,4-dichlorobenzoyl)-1-phenylsulfonylpyrrole in the form of white crystals with a melting point of 122° C.

b) 2,5'-Dichloro-2'-hydroxy-3'-methyl-4-methylsulfonylbenzophenone (Table 1, Example No. 100)

5.06 g of 2-chloro-4-methylsulfonylbenzoyl chloride are added in portions at room temperature to a mixture of 2.85 g of 4-chloro-2-methylphenol and 5.40 g of aluminum chloride in 100 ml of 1,1,2,2-tetrachloroethane, and the mixture is subsequently refluxed for 7 hours. When cold, the reaction mixture is poured onto 200 g of ice and 500 ml of concentrated hydrochloric acid. The organic phase is separated off, and the aqueous phase is extracted twice using 50 ml of dichloromethane. The combined organic phases are washed until neutral, dried over magnesium sulfate and evaporated. Recrystallization from a mixture of ethyl acetate and diisopropyl ether gives 4.2 g (60% of theory) of 2,5'-dichloro-2'-hydroxy-3'-methyl-4-methylsulfonylbenzophenone in the form of colorless crystals with a melting point of 175°-180° C.

c) 2-(2,4-Dichlorobenzoyl)-1,4-quinone (Table 1, Example No. 91)

A solution of 2.49 g of 2-(2,4-dichlorobenzoyl)-1,4-dihydroxybenzene in 20 ml of glacial acetic acid is added dropwise at 10° C. in the course of 3 minutes to a solution of 1.60 g of sodium dichromate in 2 g of concentrated sulfuric acid and 60 ml of water. The reaction mixture is stirred vigorously for a further 30 minutes at 10°-15° C. and then filtered. The residue is washed with water until free from acid and recrystallized from diisopropyl ether. 1.74 g (70% of theory) of 2-(2,4-dichlorobenzoyl)-1,4-quinone are obtained in the form of yellowish-orange crystals with a melting point of 100° C.

d) 2,2',4'-Trichloro-6'-hydroxy-4-methylsulfonylbenzophenone (Table 1, Example No. 183)

3.00 g of 2-chloro-4-methylsulfonylbenzoyl chloride are added in portions at room temperature to a mixture of 1.97 g of 3,5-dichlorophenone and 3.23 g of aluminum chloride in 50 ml of 1,1,2,2-tetrachloroethane, and the mixture is subsequently refluxed for 7 hours. When cold, the reaction mixture is poured onto 100 g of ice and 20 ml of concentrated hydrochloric acid. The organic phase is removed, and the aqueous phase is extracted twice using 50 ml of dichloromethane. The combined organic phases are washed until neutral and subsequently extracted three times using in each case 25 ml of 2N sodium hydroxide solution. The combined alkaline extracts are brought to a pH of 2 using concentrated hydrochloric acid. The precipitate is filtered off, washed with water until neutral and dried. This gives 3.194 g (70% of theory) of 2,2',4'-trichloro-6'-hydroxy-4-methylsulfonylbenzophenone in the form of colorless crystals with a melting point of 123°-126° C.

The compounds listed in Table I are obtained analogously to the preparation examples described above.

                                      TABLE I                                      __________________________________________________________________________     Compounds of the formula (I) according to the invention                         ##STR7##                                                                      No.                                                                               R.sup.1                                                                             R.sup.2                                                                              R.sup.3                                                                             Q             M.p.  °C.!                                                                    IC.sub.50                               __________________________________________________________________________     1  Cl   H     NO.sub.2                                                                             ##STR8##     135-139                                                                              <10.sup.-9                              2  Cl   CH.sub.3                                                                             SO.sub.2 CH.sub.3                                                                    ##STR9##                                                   3  Cl   H     OCF.sub.2 H                                                                          ##STR10##                                                  4  Br   H     CN                                                                                   ##STR11##                                                  5  Br   H     NO.sub.2                                                                             ##STR12##                                                  6  Cl   OC.sub.2 H.sub.5                                                                     SO.sub.2 CH.sub.3                                                                    ##STR13##                                                  7  Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR14##    228   5.7 × 10.sup.-7                   8  F    OH    CH.sub.2 CF.sub.3                                                                    ##STR15##                                                  9  NO.sub.2                                                                            F     CH.sub.2 Cl                                                                          ##STR16##                                                  10 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR17##    225 (decomp.)                                                                        1.8 × 10.sup.-7                   11 OC.sub.2 H.sub.5                                                                    CF.sub.2 H                                                                           SO.sub.2 CH.sub.3                                                                    ##STR18##                                                  12 OCH.sub.3                                                                           H     Br                                                                                   ##STR19##                                                  13 OH   Cl    Cl                                                                                   ##STR20##    98-100                                                                               <10.sup.-9                              14 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR21##    267 (decomp.)                                 15 F    COOCH.sub.3                                                                          CH.sub.2 CF.sub.3                                                                    ##STR22##                                                  16 Cl   H     Cl                                                                                   ##STR23##    147                                           17 Cl   Cl    Br                                                                                   ##STR24##                                                  18 CN   F     CH.sub.2 Cl                                                                          ##STR25##                                                  19 OH   Cl    Cl                                                                                   ##STR26##    95-101                                        20 CH.sub.2 Cl                                                                         OCH.sub.3                                                                            NO.sub.2                                                                             ##STR27##                                                  21 Br   Cl    Cl                                                                                   ##STR28##                                                  22 OH   Cl    Cl                                                                                   ##STR29##    126-128                                       23 H    H     SO.sub.2 CH.sub.3                                                                    ##STR30##    224                                           24 CH.sub.2 CF.sub.3                                                                   COOCH.sub.3                                                                          Cl                                                                                   ##STR31##                                                  25 F    Cl    SO.sub.2 CH.sub.3                                                                    ##STR32##                                                  26 OH   Cl    Cl                                                                                   ##STR33##    97-100                                        27 NO.sub.2                                                                            H     CF.sub.3                                                                             ##STR34##                                                  28 Br   Cl    OCF.sub.2                                                                            ##STR35##                                                  29 Cl   H     NO.sub.2                                                                             ##STR36##    192                                           30 NO.sub.2                                                                            H     Cl                                                                                   ##STR37##                                                  31 CCl.sub.3                                                                           F     SO.sub.2 CH.sub.3                                                                    ##STR38##                                                  32 Cl   H     NO.sub.2                                                                             ##STR39##    178                                           33 NO.sub.2                                                                            Cl    Cl                                                                                   ##STR40##                                                  34 OH   Cl    Cl                                                                                   ##STR41##    124-128                                       35 Cl   H     Cl                                                                                   ##STR42##    135   < × 10.sup.-9                     36 Cl   Cl    CN                                                                                   ##STR43##                                                  37 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR44##                                                  38 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR45##    147                                           39 Cl   COOCH.sub.3                                                                          SO.sub.2 CH.sub.3                                                                    ##STR46##                                                  40 Cl   H     Cl                                                                                   ##STR47##    86    9.7 × 10.sup.-7                   41 Cl   C.sub.3 H.sub.7                                                                      SO.sub.2 CH.sub.3                                                                    ##STR48##                                                  42 OH   Cl    Cl                                                                                   ##STR49##    134-136                                                                              3.5 × 10.sup.-6                   43 Cl   H     Cl                                                                                   ##STR50##    103   3.5 × 10.sup.-6                   44 H    OCF.sub.3                                                                            F                                                                                    ##STR51##                                                  45 NO.sub.2                                                                            H     Cl                                                                                   ##STR52##    125                                           46 Cl   CH.sub.3                                                                             CF.sub.2 H                                                                           ##STR53##                                                  47 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR54##    163-166                                                                              5.7 × 10.sup.-6                   48 F    COOCH.sub.3                                                                          OCF.sub.2 H                                                                          ##STR55##                                                  49 Cl   H     Cl                                                                                   ##STR56##    99                                            50 OH   Cl    Cl                                                                                   ##STR57##    98-103                                        51 CH.sub.2 Cl                                                                         C.sub.3 H.sub.7                                                                      SO.sub.2 CH.sub.3                                                                    ##STR58##                                                  52 NO.sub.2                                                                            OH    Br                                                                                   ##STR59##                                                  53 CF.sub.3                                                                            H     Cl                                                                                   ##STR60##                                                  54 Cl   H     Cl                                                                                   ##STR61##                                                  55 Cl   Cl    Cl                                                                                   ##STR62##                                                  56 OH   Cl    Cl                                                                                   ##STR63##    64-65 5.0 × 10.sup.-8                   57 Br   H     OCF.sub.3                                                                            ##STR64##                                                  58 F    CH.sub.3                                                                             F                                                                                    ##STR65##                                                  59 Br   Cl    Cl                                                                                   ##STR66##                                                  60 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR67##    165                                           61 NO.sub.2                                                                            H     OCF.sub.2 H                                                                          ##STR68##                                                  62 CF.sub.3                                                                            Cl    Cl                                                                                   ##STR69##                                                  63 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR70##    227                                           64 CCl.sub.3                                                                           H     Cl                                                                                   ##STR71##                                                  65 Cl   H     CF.sub.3                                                                             ##STR72##                                                  66 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR73##    228                                           67 Cl   CH.sub.3                                                                             OCF.sub.2 H                                                                          ##STR74##                                                  68 Cl   CF.sub.3                                                                             OCF.sub.2 H                                                                          ##STR75##                                                  69 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR76##                                                  70 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR77##    240 (decomp.)                                                                        4.0 × 10.sup.-8                   71 OCH.sub.3                                                                           COOCH.sub.3                                                                          SO.sub.2 CH.sub.3                                                                    ##STR78##                                                  72 Cl   H     SO.sub.2 C.sub.2 H.sub.5                                                             ##STR79##                                                  73 CF.sub.3                                                                            CH.sub.3                                                                             Cl                                                                                   ##STR80##                                                  74 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR81##    151-153                                       75 F    OCF.sub.3                                                                            Cl                                                                                   ##STR82##                                                  76 Cl   CF.sub.3                                                                             Cl                                                                                   ##STR83##                                                  77 H    COOC.sub.2 H.sub.5                                                                   SO.sub.2 CH.sub.3                                                                    ##STR84##                                                  78 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR85##                                                  79 NO.sub.2                                                                            H     Cl                                                                                   ##STR86##    160-162                                       80 F    Cl    Cl                                                                                   ##STR87##                                                  81 OCH.sub.3                                                                           COOCH.sub.3                                                                          SO.sub.2 CH.sub.3                                                                    ##STR88##                                                  82 Cl   Cl    OH                                                                                   ##STR89##    137-139                                       83 Cl   Cl    CF.sub.3                                                                             ##STR90##                                                  84 F    C.sub.2 H.sub.5                                                                      Cl                                                                                   ##STR91##                                                  85 OCF.sub.2 H                                                                         Cl    Cl                                                                                   ##STR92##                                                  86 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR93##    157                                           87 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR94##    108   2.4 × 10.sup.-7                   88 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR95##    254-256                                                                              7.0 × 10.sup.-8                   89 CF.sub.2 H                                                                          Cl    Cl                                                                                   ##STR96##                                                  90 Br   H     SO.sub.2                                                                             ##STR97##                                                  91 Cl   H     Cl                                                                                   ##STR98##    100   1.8 × 10.sup.-8                   92 Cl   C.sub.2 H.sub.5                                                                      CF.sub.3                                                                             ##STR99##                                                  93 F    F     F                                                                                    ##STR100##                                                 94 Cl   Cl    OH                                                                                   ##STR101##   149-152                                       95 OH   Cl    Cl                                                                                   ##STR102##   118-121                                       96 OC.sub.2 H.sub.5                                                                    Cl    CF.sub.3                                                                             ##STR103##                                                 97 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR104##   180 (decomp.)                                                                        6.0 × 10.sup.-8                   98 CN   F     F                                                                                    ##STR105##                                                 99 Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR106##                                                 100                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR107##   175-180                                                                              6.2 × 10.sup.-9                   101                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR108##   183   <10.sup.-9                              102                                                                               Cl   OCH.sub.3                                                                            NO.sub.2                                                                             ##STR109##                                                 103                                                                               F    Cl    Cl                                                                                   ##STR110##                                                 104                                                                               Cl   H     Cl                                                                                   ##STR111##   90                                            105                                                                               Cl   H     SO.sub.2 C.sub.2 H.sub.5                                                             ##STR112##                                                 106                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR113##   141-143                                       107                                                                               H    COOC.sub.2 H.sub.5                                                                   CN                                                                                   ##STR114##                                                 108                                                                               OCH.sub.3                                                                           H     CF.sub.3                                                                             ##STR115##                                                 109                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR116##   171-175                                                                              <10.sup.-8                              110                                                                               H    COOC.sub.2 H.sub.5                                                                   CF.sub.3                                                                             ##STR117##                                                 111                                                                               CCl.sub.3                                                                           Cl    Cl                                                                                   ##STR118##                                                 112                                                                               CCl.sub.3                                                                           Cl    Cl                                                                                   ##STR119##                                                 113                                                                               CF.sub.3                                                                            OCH.sub.3                                                                            Cl                                                                                   ##STR120##                                                 114                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR121##   234-235                                                                              9.0 × 10.sup.-9                   115                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR122##   98-100                                        116                                                                               Cl   Cl    Br                                                                                   ##STR123##                                                 117                                                                               CN   H     Cl                                                                                   ##STR124##                                                 118                                                                               Cl   CF.sub.3                                                                             Cl                                                                                   ##STR125##                                                 119                                                                               NO.sub.2                                                                            OC.sub.2 H.sub.5                                                                     Cl                                                                                   ##STR126##                                                 120                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR127##   182-187                                                                              <10.sup.-9                              121                                                                               F    OCH.sub.3                                                                            F                                                                                    ##STR128##                                                 122                                                                               Cl   Cl    Cl                                                                                   ##STR129##                                                 123                                                                               CF.sub.3                                                                            COOCH.sub.3                                                                          Cl                                                                                   ##STR130##                                                 124                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR131##   188-192                                       125                                                                               Cl   COOC.sub.2 H.sub.5                                                                   Cl                                                                                   ##STR132##                                                 126                                                                               NO.sub.2                                                                            H     Cl                                                                                   ##STR133##                                                 127                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR134##   136-140                                       128                                                                               OH   CF.sub.2 H                                                                           Br                                                                                   ##STR135##                                                 129                                                                               F    CH.sub.3                                                                             SO.sub.2 CH.sub.3                                                                    ##STR136##                                                 130                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR137##   198-202                                                                              1.0 × 10.sup.-9                   131                                                                               Cl   H     CF.sub.3                                                                             ##STR138##                                                 132                                                                               Cl   H     Cl                                                                                   ##STR139##                                                 133                                                                               H    Cl    Cl                                                                                   ##STR140##                                                 134                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR141##   278                                           135                                                                               Cl   OC.sub.2 H.sub.5                                                                     C.sub.2 F.sub.5                                                                      ##STR142##                                                 136                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR143##   105-108                                                                              3.0 × 10.sup.-7                   137                                                                               C.sub.2 F.sub.5                                                                     COOCH.sub.3                                                                          Cl                                                                                   ##STR144##                                                 138                                                                               Cl   H     Cl                                                                                   ##STR145##   173                                           139                                                                               CN   F     Cl                                                                                   ##STR146##                                                 140                                                                               H    Cl    SO.sub.2 CH.sub.3                                                                    ##STR147##                                                 141                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR148##   182-185                                                                              1.1 × 10.sup.-9                   142                                                                               Cl   CF.sub.2 H                                                                           SO.sub.2 C.sub.2 H.sub.5                                                             ##STR149##                                                 143                                                                               OC.sub.2 H.sub.5                                                                    COOCH.sub.3                                                                          SO.sub.2 CH.sub.3                                                                    ##STR150##                                                 144                                                                               CF.sub.3                                                                            Cl    SO.sub.2 CH.sub.3                                                                    ##STR151##                                                 145                                                                               NO.sub.2                                                                            Cl    OCF.sub.2 H                                                                          ##STR152##                                                 146                                                                               Cl   H     Cl                                                                                   ##STR153##   178                                           147                                                                               CH.sub.2 Cl                                                                         Cl    SO.sub.2 CH.sub.3                                                                    ##STR154##                                                 148                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR155##   193-195                                                                              9.2 × 10.sup.-7                   149                                                                               CF.sub.2 H                                                                          H     OCF.sub.3                                                                            ##STR156##                                                 150                                                                               Cl   H     Cl                                                                                   ##STR157##   122                                           151                                                                               F    CF.sub.3                                                                             F                                                                                    ##STR158##                                                 152                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR159##   143-144                                                                              1.2 × 10.sup.-7                   153                                                                               Cl   Cl    Cl                                                                                   ##STR160##                                                 154                                                                               Cl   COOCH.sub.3                                                                          CF.sub.3                                                                             ##STR161##                                                 155                                                                               C.sub.2 F.sub.5                                                                     H     NO.sub.2                                                                             ##STR162##                                                 156                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR163##   160-165                                                                              <10.sup.-9                              157                                                                               CF.sub.3                                                                            H     F                                                                                    ##STR164##                                                 158                                                                               Cl   CF.sub.3 H                                                                           SO.sub.2 C.sub.2 H.sub.5                                                             ##STR165##                                                 159                                                                               C.sub.2 F.sub.5                                                                     H     Cl                                                                                   ##STR166##                                                 160                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR167##   142-145                                                                              2.3 × 10.sup.-7                   161                                                                               CCl.sub.3                                                                           H     SO.sub.2 CH.sub.3                                                                    ##STR168##                                                 162                                                                               Cl   CF.sub.3                                                                             Cl                                                                                   ##STR169##                                                 163                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR170##   167-170                                                                              1.2 × 10.sup.-8                   164                                                                               Br   OCF.sub.2 H                                                                          CF.sub.3                                                                             ##STR171##                                                 165                                                                               Cl   OCH.sub.3                                                                            SO.sub.2 CH.sub.3                                                                    ##STR172##                                                 166                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR173##   154-155                                                                              2.0 × 10.sup.-8                   167                                                                               H    F     CCl.sub.3                                                                            ##STR174##                                                 168                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR175##   161                                           169                                                                               CH.sub.2 CF.sub.3                                                                   Cl    Cl                                                                                   ##STR176##                                                 170                                                                               CF.sub.3 H                                                                          OCH.sub.3                                                                            CN                                                                                   ##STR177##                                                 171                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR178##   228-230                                                                              2.2 × 10.sup.-8                   172                                                                               CF.sub.3                                                                            H     Cl                                                                                   ##STR179##                                                 173                                                                               Cl   H     Cl                                                                                   ##STR180##   92                                            174                                                                               Br   OCF.sub.3                                                                            CH.sub.2 Cl                                                                          ##STR181##                                                 175                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR182##   195-196                                                                              1.2 × 10.sup.-8                   176                                                                               OH   COOC.sub.2 H.sub.5                                                                   CF.sub.2 H                                                                           ##STR183##                                                 177                                                                               H    OCH.sub.3                                                                            F                                                                                    ##STR184##                                                 178                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR185##   118-120                                                                              4.0 × 10.sup.-9                   179                                                                               Br   OCH.sub.3                                                                            CF.sub.3                                                                             ##STR186##                                                 180                                                                               NO.sub.2                                                                            COOCH.sub.3                                                                          OCF.sub.2 H                                                                          ##STR187##                                                 181                                                                               Cl   H     Cl                                                                                   ##STR188##   134                                           182                                                                               CN   F     CH.sub.2 CF.sub.3                                                                    ##STR189##                                                 183                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR190##   123-125                                                                              4.8 × 10.sup.-9                   184                                                                               Br   OCH.sub.3                                                                            CF.sub.2 H                                                                           ##STR191##                                                 185                                                                               Cl   OCF.sub.2 H                                                                          NO.sub.2                                                                             ##STR192##                                                 186                                                                               OC.sub.2 H.sub.5                                                                    COOC.sub.2 H.sub.5                                                                   Cl                                                                                   ##STR193##                                                 187                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR194##   117-121                                                                              4.0 × 10.sup.-7                   188                                                                               F    Cl    CCl.sub.3                                                                            ##STR195##                                                 189                                                                               Cl   H     Cl                                                                                   ##STR196##   101                                           190                                                                               Cl   OCF.sub.3                                                                            SO.sub.2 CH.sub.3                                                                    ##STR197##                                                 191                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR198##   164-166                                                                              <10.sup.-8                              192                                                                               Br   OH    CF.sub.3                                                                             ##STR199##                                                 193                                                                               OH   Cl    Cl                                                                                   ##STR200##   116-118                                       194                                                                               Cl   Cl    OH                                                                                   ##STR201##   69-71                                         195                                                                               Br   H     CCl.sub.3                                                                            ##STR202##                                                 196                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR203##   162-168                                                                              <10.sup.-9                              197                                                                               F    H     Br                                                                                   ##STR204##                                                 198                                                                               Cl   H     Cl                                                                                   ##STR205##   132                                           199                                                                               Cl   Br    CF.sub.3                                                                             ##STR206##                                                 200                                                                               Cl   H     Cl                                                                                   ##STR207##   135                                           201                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR208##   115-118                                                                              <10.sup.-9                              202                                                                               F    COOCH.sub.3                                                                          SO.sub.2 CH.sub.3                                                                    ##STR209##                                                 203                                                                               CH.sub.2 CF.sub.3                                                                   H     Br                                                                                   ##STR210##                                                 204                                                                               Cl   H     SO.sub.2 CH.sub.3                                                                    ##STR211##   118-121                                                                              <10.sup.-5                              __________________________________________________________________________

The invention furthermore relates to the use of the herbicidal active substances identified by means of the assay method according to the invention in plant populations.

Suitable plant populations are, in particular, crop plants.

In the examples which follow, the invention is described further and illustrated in greater detail with the aid of examples. Percentages are by weight unless otherwise specified.

Example 1 Enriching HPPD from Maize Plants

To enrich the anabolic HPPD from plant tissues, young etiolated maize plants were homogenized in extraction buffer (20 mM phosphate buffer pH 7.0; 0.14M KCl; 0.1 mg/ml glutathione and 1% insoluble polyvinylpyrrolidone) for 3×10 seconds, for example in a Waring blender. The extract was filtered (all other isolation steps were carried out at 4° C.) and centrifuged (20 minutes; 10,000 g). The supernatant was subjected to fractional ammonium sulfate precipitation. At from 20 to 40% saturation of the solution with ammonium sulfate, HPPD precipitated. At this stage, it was possible to store the enzyme at -80° C. following dialysis in a suitable buffer, such as, for example, 20 mM phosphate buffer pH 7.0; 0.14M KCl; 0.1 mg/ml glutathione. This degree of purity of the enzyme is sufficient for identifying inhibitors.

Example 2 Method for the Identification of Potential HPPD Inhibitors

The enzyme assays were performed in 20 ml polyethylene screw-top containers. Two hypodermic needles (0.9×40 mm) were pushed through the screw-top cap. One of the hypodermic needles was bent to form a hook onto which a small filter paper disk of diameter 10 mm was placed. The filter paper was moistened with 25 μl of methylbenzethonium hydroxide (Sigma). The second hypodermic needle was used for injecting start and stop solution into the screw-top container.

The enzyme reaction mix contained:

1 ml of 0.1M phosphate buffer, pH 7.3;

2 g/l of bovine liver catalase;

100 μl of a mixture of 3 mM dichlorophenolindophenol and

150 mM glutathione (reduced);

25 μl of acetone (or potential inhibitors dissolved in acetone) and

enzyme in a total volume of 1,635 μl.

After preincubation for 5 minutes at 30° C., the reaction was started by injecting 800 μl of ¹⁴ C-p-hydroxyphenylpyruvate. The reaction was stopped by injecting 600 μl of 1N H₂ SO₄, incubation was continued for 15 minutes, and the radioactivity which was liberated was determined on the small filter paper disks in a liquid scintillation counter.

Example 3 Demonstration of the Inhibition of Plastoquinone Biosynthesis in Wheat by SC-0051

Thin-layer chromatography analysis of the incorporation of radioactivity from ¹⁴ C-tyrosine into plastoguinone. Wheat plants were either not treated (A), or sprayed with a standard screening formulation (B) or with 100 g/ha of SC-0051 in this screening formulation (C), and then incubated with ¹⁴ C-tyrosine. The lipophilic metabolites of the wheat were extracted using chloroform, separated on silica gel 60 HPTLC plates using chloroform: diethyl ether (99:1), and the radioactivity was analyzed by thin-layer scanner. The incorporation of radioactivity from tyrosine into plastoquinone (see FIG. 1, band 2) can be seen clearly in the control batches (A) and (B), while plastoquinone biosynthesis is inhibited by the use of SC-0051 (C).

Example 4 Inhibition of Partially Purified Maize HPPD by the Herbicide SC-0051

The test was carried out analogously to Example 3, but a different inhibitor which also results in bleaching symptoms was used (clomazone). While SC-5001 shows a marked inhibition of HPPD (o), the other bleaching herbicide has no inhibitory action on HPPD () (see FIG. 2).

Example 5 Demonstration of the Correlation between HPPD Inhibition and Herbicidal Activity

HPPD catalyzes the transformation of p-hydroxyphenylpyruvate to homogentisic acid. If HPPD inhibition is responsible for the herbicidal activity of an inhibitor, then it should be possible to neutralize the herbicidal effect by supplementation with homogentisic acid. This was demonstrated by an experiment with the dockweed Lemna gibba. The plant was grown in a sterile aqueous medium supplemented as described in Table 1.

Table II: Neutralization of the effect of SC-0051 by homogentisic acid

    ______________________________________                                         Supplementation                                                                               Number of L. gibba plants                                       of the medium  Control 10.sup.-7 M SC-0051                                     ______________________________________                                         --             25      3                                                       1 mM HPPD      22      2                                                       1 mM HGA       29      32                                                      ______________________________________                                    

Batches of three Lemna plants in 100 ml of sterile medium were incubated for six days with sterile-filtered supplements. SC-0051 was added in the form of a 10⁻³ M stock solution (active substance in acetone). The number of newly formed plants was determined on day six.

Example 6 Effect of Benzophenones

    ______________________________________                                                              Growth inhibition of                                      Example No.          Lemna (%)                                                 from Table I                                                                             IC.sub.50 value (M)                                                                       10.sup.-4 M                                                                              10.sup.-5 M                                                                          10.sup.-6 M                               ______________________________________                                         183       4.8 × 10.sup.-9                                                                     98        82    81                                        160       2.3 × 10.sup.-7                                                                     80        72    12                                        204       >10.sup.-5 28        12    --                                        171       2.2 × 10.sup.-8                                                                     98        93    93                                        100       6.2 × 10.sup.-9                                                                     98        96    92                                        ______________________________________                                    

Table 2 shows IC₅₀ values of p-hydroxyphenylpyruvate dioxygenase inhibition by a range of benzophenone derivatives in comparison with growth inhibition of Lemna gibba.

The IC₅₀ value is the molar concentration of the inhibitor at which 50% of the enzyme activity are inhibited. The growth of Lemna was measured as described in: Schulz, A., Ort, O., Beyer, P. and Kleinig, H.; SC-0051, a 2-benzoylcyclohexane-1,3-dione bleaching herbicide, is a potent inhibitor of the enzyme p-hydroxyphenylpyruvate dioxygenase. FEBS-Letters, 1993, 318 (2), 162-166.

It can be seen from this table that the herbicidal activity of the benzophenone derivatives is correlated with the in vitro activity. 

I claim:
 1. A p-hydroxyphenylpyruvate dioxygenase inhibitor identified by the general formula (I) ##STR212## wherein: R¹ is Cl,R² is H, R³ is methylsulfonyl, and Q is 2,4-dichloro-6-hydroxyphenyl.
 2. A herbicidal composition containing a p-hydroxyphenylpyruvate dioxygenase inhibitor as claimed in claim
 1. 